1. Technical Field
This invention relates to a heat resistant coated member which is used in the sintering or heat treatment of powder metallurgical metal, cermet or ceramic materials in vacuum or an inert or reducing atmosphere; a method for preparing the same; and a method for the heat treatment of powder metallurgical metal, cermet or ceramic materials using the coated member.
2. Background Art
Powder metallurgy products are generally manufactured by mixing a primary alloy with a binder phase-forming powder, then kneading the mixture, followed by compaction, sintering and post-treatment. The sintering step is carried out in a vacuum or an inert gas atmosphere, and at an elevated temperature of 1,000 to 1,600° C.
In a typical cemented carbide manufacturing process, solid solutions of tungsten carbide with cobalt, titanium carbide, and tantalum carbide are comminuted and mixed, then subjected to drying and granulation to produce a granulated powder. The powder is then pressed, following which such steps as dewaxing, pre-sintering, sintering and machining are carried out to give the final cemented carbide product.
Sintering is carried out at or above the temperature at which the cemented carbide liquid phase appears. For example, the eutectic temperature for a ternary WC—Co system is 1,298° C. The sintering temperature is generally within a range of 1,350 to 1,550° C. In the sintering step, it is important to control the atmosphere so that cemented carbide correctly containing the target amount of carbon may be stably sintered.
When cemented carbide is produced by sintering at about 1,500° C., green specimens placed on a carbon tray often react with the tray. That is, a process known as carburizing occurs, in which carbon from the tray impregnates the specimen, lowering the strength of the specimen. A number of attempts have been made to avoid this type of problem, either by choosing another type of tray material or by providing on the surface of the tray a barrier layer composed of a material that does not react with the green specimen. For example, ceramic powders such as zirconia, alumina and yttria are commonly used when sintering cemented carbide materials. One way of forming a barrier is to scatter the ceramic powder over the tray and use it as a placing powder. Another way is to mix the ceramic powder with a solvent and spray-coat the mixture onto the tray or apply it thereto as a highly viscous slurry. Yet another way is to form a coat by using a thermal spraying or other suitable process to deposit a dense ceramic film onto the tray. Providing such an oxide layer as a barrier layer on the surface of the tray has sometimes helped to prevent reaction of the tray with the specimen.
In general, the powder metallurgy or ceramic manufacturing process involves firing or sintering and heat treatment steps. The specimen that is to become a product is set on the tray. Since the specimen can react with the tray material to invite a deformation or compositional shift or introduce impurities into the product, there are many cases where products are not fired or sintered in high yields. There are many ways for preventing the reaction of the tray with the product, as described above. For example, an oxide powder such as alumina or yttria or a nitride powder such as aluminum nitride or boron nitride is used as the placing powder. Alternatively, such an oxide or nitride powder is mixed with an organic solvent to form a slurry, which is coated or sprayed to the tray to form a coating on the tray for preventing the tray from reacting with the product. On use of placing powder, however, some of the placing powder will deposit on the product. The slurry coating procedure must be repeated every one or several sintering steps because the coating peels from the substrate (tray).
To solve these problems, JP-A 2000-509102 proposes to form a dense coating on the surface of a tray by a thermal spraying technique. Specifically, when a graphite tray is used in the sintering of materials to produce cemented carbides or cermets, the graphite tray is coated with a cover layer made of Y2O3 containing up to 20% by weight of ZrO2 or an equivalent volume of another heat resistant oxide such as Al2O3 or a combination thereof, and having an average thickness of at least 10 μm.
Although the thermally sprayed coating of this patent publication is effective for preventing reaction with the product, there is a likelihood that the coating readily peels off due to thermal degradation at the interface between the coating and the tray substrate by repeated thermal cycling. It is thus desired to have a coated member in which the oxide coating does not peel from the substrate even when subjected to repeated thermal cycling, that is, having heat resistance, corrosion resistance, durability and non-reactivity.
More particularly, even when a barrier layer is formed on a carbon tray, reaction can occur between the barrier layer and the tray. After one or a few sintering cycles, the barrier layer cracks, fragments and spalls off. Peeling of the coating allows for reaction between the carbon tray and a specimen. During the sintering step, the coating can peel and fragment into pieces which are often introduced into the specimen. Then a fresh coated tray must be used.
For the above-described reason, there is a need for a tray having a long lifetime in that when used in sintering, the barrier layer does not react with a specimen or with the tray substrate or peel off, and when used in the sintering of powder metallurgical products, the barrier layer does not react with specimens or peel from the tray substrate even after repeated use.